1. Field of the Invention
The present invention relates to photolithographic techniques in semiconductor processing, and, more particularly, to high resolution optical lithography using the g-, h-, or i-lines of the mercury spectrum for exposure of photolithographic films on a highly reflective surface, such as aluminum or titanium.
2. Description of the Related Art
In the construction of integrated circuit devices, one or more metallic layers, such as aluminum or titanium, are deposited and subsequently patterned to provide ohmic or Schottky contacts and electrical connections between various circuit elements. Conventionally, photoresist is spun over the metallic layer and then exposed to a light pattern and developed. The normally aluminum metallic layer is then selectively plasma-etched with chlorine-containing gases through the openings in the resist layer. The remaining photoresist is then removed, leaving the final metal pattern.
Antireflective coatings have been used over reflective surfaces such as aluminum or titanium to improve resist patterning control by reducing interference effects and diffuse scattering, particularly if the illumination is monochromatic. Such effects have become less and less tolerable as linewidth and pitch have been reduced in newer integrated circuit designs of greater density.
One approach to reduce the reflected light has been to use metal or refractory materials as antireflective films on aluminum. Examples of such materials include titanium-tungsten (TiW) and molybdenum disilicide (MoSi.sub.2). Van den Berg et al in "Antireflective Coatings on Metal Layers for Photolighographic Purposes", Journal of Applied Physics, Vol. 50 , No. 3, pp. 1212-1214 (March 1979) proposed the use of amorphous silicon, amorphous selenium and chromium oxide as antireflective coatings. However, amorphous silicon reduces aluminum reflectance at a wavelength 436 nm to only about 15 to 20%, and its deposition characteristically intoduces numerous harmful particulates.
The use of organic coatings, such as a thin polyimide with a light absorbing dye, has been described by Brewer et al in "The Reduction of the Standing Wave Effect in Positive Photoresists", Journal of Applied Photographic Engineering, Vol. 7, No. 6, pp. 184-186 (December 1981). However, this technique requires a separate polyimide spin and cure. The curing technique has proved difficult to control properly on the production line because of the tight temperature tolerances required.